As to what you should get, it depends on what sort of games you want to play. If you are going to be playing tournament games and want to push for the win I would select from this list:Wraithknights,Windriders (Jetbikes)Warp Spiders Other things you need to make a tournament legal army.

If you are playing friendlier games you can take pretty much anything in the eldar army as they are all pretty good in relation to most things in other codexes. Dark Reapers, Wraithguard in a serpent, Dire Avengers, Crimson Hunters, War Walkers, all varieties of grav tanks + others are all easily strong enough for most friendly games.

On top of that there is the new Ynnari guys too, meaning you can take basically anything from Eldar, Dark Eldar and Harlequins.

You started Eldar because you liked one model. Why not continue like this. Take what you like, fun is the name of the game. You can't really go wrong with Eldar.

Only taking the strongest units will let you win more often, but you will be starved to find games too. There are basicly no bad units in the Eldar faction. Especially now that you can take assault/open topped vehicles for your units that needed them.

Not saying you shouldn't build the strongest possible army, that's a totally ok thing to do. Just be careful of the consequences.

is a full biker army any good? The conversions that I have in mind will work good on bikers. So I would go for a biker army with the new avatar and maybe a wraith knight.

Windriders are crazy cheese. Fast, evasive, objective securing, heavy weapon platforms. With the new Soulburst rule where if something dies near them, they get a free out of sequence action. Windriders are so good that even before Gathering Storm part Eldar was a thing, I wanted to make an army of only them, Farseer Skyrunners, and Hornets from Forgeworld. Stupid amounts of shooting.

That are 2 lists that i personally would use at the 500 points mark, you don't need that much anti tank capabilities because you won't see heavy AV that often and if you do the Spears from the craftworld list have lance weapons, just in case ^^°